New PI technology proposals.

[ZED]

question

Aziz in an effort to see where your coming from in regard to the fly back i have a question,
The attatched scope pic is of a target signal taken straight of the receive coil.

Setup
1/2 inch round copper tubing bent into a circle of 18 inches diameter with ends not connected together,used as transmit coil.
Two receive coils residing within transmit coil,approx 30 turns of wire per receive coil,receive coils connected opposite in phase to cancle out flyback signal.
12 volts on transmit,45 amps running through coil,max current reached within 10u/s,pulse width 34u/s


Question
if i understand you correctly you say the flyback kick starts the eddy currents on the target.
on the scope pick,at the start of the transmit,while current is rising very fast i get a very good signal on the target,yet there is no flyback to generate this high target signal,are you able to explain why this is so???

Zed

[Aziz]

Important notice:

I have the proof of that my assumptions aren't correct! A new SPICE simulation shows exactly, what is going in the system.


Bbsailor, your idea is working extremely good!

Aziz

[Aziz]

BTW Carl,

an existing DD coil configuration front-end can be easily used for tests. The difference amplifier part could then be omitted.

Aziz

[Aziz]

Hi Carl,

I don't think Aziz is disagreeing with this. Yes, it is dB/dt that induces eddy currents in the target. And it is di/dt that produces dB/dt. I think our only disagreement is with the flyback voltage... it is an artifact of the system.

I don't disagree with you what physics laws stating. I only disagree with the name "artifact of the system". Faraday shouldn't here this. We could call this just flyback voltage induction. Some people could get the feeling of an useless thing. But the opposite occurs in the PI technology. We are using this so called "artifact" for detection.

If Aziz is seeing a higher flyback that is settling faster, then I expect it is due to a faster di/dt and the arrangement of the mutually inductive coils. I doubt it is due to the higher voltage burning off the current faster. I think it is this cause & effect where we disagree.

But that's OK. The main thing is, does it Really Work? I will have an answer tomorrow. I am (right now) winding the two coils -- one center-tapped & one not -- to compare this method with the std mono. I have the mono wound and hot-glued into a TDI shell, will have the CT coil done tonight. Then I will put them on my HH board and see.

Stay tuned...

- Carl

I am also very interested in practical results and looking forward to it. Particularly, when there are effects not taken into account or to be reconsidered. It could give us a chance to dig a bit deeper or totally recapitulating the thoughts and start again at 0.

Regards,
Aziz

[1843]

If Aziz is seeing a higher flyback that is settling faster, then I expect it is due to a faster di/dt and the arrangement of the mutually inductive coils. I doubt it is due to the higher voltage burning off the current faster. I think it is this cause & effect where we disagree.


- Carl

Hi,

Using a higher voltage MOSFET cuases a faster settling down. I have tried this many times and that's why the designer of www.pulsdetektor.de uses IGBTs.(1200V)

Aziz doesn't say we make the flyback voltage higher... but he says we don't limit it by a MOSFET's internal diode.

The reason for this is that when the flyback voltage goes higher than the MOSFET's breakdown voltage, the MOSFET's clamping diode is shorted and then the current flows through the diode rather than the damping resistor....

In the PIs like HH, we cannot make setteling down faster because the flyback voltage is under the MOSFET's breakdown voltage.



I hope this helps
1843

[Aziz]

Hi guys,

I could perhaps make some mistakes in my thoughts. It is difficult to compare two different configurations. Fact is, there is due to center-tapped coil the twice target signal stimulation (measureable). This is due to higher dI/dt during the flyback damping process. This stimulation will arrive the target and then back. So the target response on the receive coil should not a factor of twice of course.

I suggest to make only performance tests (what counts tests). Giving a fixed amplification factor and seeing, when the early sample time it is. The earlier, the better the sensitivity it should have. This makes the tests easy without getting confused much.

Even, when there is possibly no significant faster decaying available, what for benefits it could else have? I will not be worry about, if I have some fundamental mistakes on my thoughts.

This debate is showing purely, that not everybody has the same understanding of PI technology. So it is a big chance for us, to find a good solution and going further.

Now, what for mistakes I have made? Just feel free to correct me.

Aziz

[Carl-NC]

Yes, it seems you and Aziz believe in collapsing field the main driver of target currents -- but I saw dissenting opinions, just wondering if proven one or other.

Physically, it has to be so... induction is only caused by a changing magnetic field, not a static one. What other opinions are there?

PS. Has anyone experimented with "assymetric" damping? In other words, use a diode and have an underdamped (very fast) first half of cycle (let it start ringing), but in the next half cycle, couple in more resistance with a diode to critically damp it out? Or something like that.

I've experimented with switched damping, that is, where the damping resistor is switched in during the flyback. I did this in my security wand, strictly for lower power.

- Carl

[simonbaker]

I don't think Aziz is disagreeing with this. Yes, it is dB/dt that induces eddy currents in the target. And it is di/dt that produces dB/dt. I think our only disagreement is with the flyback voltage... it is an artifact of the system.

If Aziz is seeing a higher flyback that is settling faster, then I expect it is due to a faster di/dt and the arrangement of the mutually inductive coils. I doubt it is due to the higher voltage burning off the current faster. I think it is this cause & effect where we disagree.

But that's OK. The main thing is, does it Really Work? I will have an answer tomorrow. I am (right now) winding the two coils -- one center-tapped & one not -- to compare this method with the std mono. I have the mono wound and hot-glued into a TDI shell, will have the CT coil done tonight. Then I will put them on my HH board and see.

Stay tuned...

- Carl

Yes, it seems you and Aziz believe in collapsing field the main driver of target currents -- but I saw dissenting opinions, just wondering if proven one or other.

Look forward to your experiments!

Regards,

-SB

PS. Has anyone experimented with "assymetric" damping? In other words, use a diode and have an underdamped (very fast) first half of cycle (let it start ringing), but in the next half cycle, couple in more resistance with a diode to critically damp it out? Or something like that.

[simonbaker]

Some hints for experimentators:

To get the proof of the earlier decaying, keep in mind, that the center-tapped coil will produce twice more signal levels! This means, that the target signal is already inherently amplified with factor of 2. This applies also to the decay curve.

To compare the proposed design with conventional design, use a conventional design with equal conditions but amplifing the RX signal twice. Then exact timings can be measured and compared.


Aziz

Something I'm not clear about:

Battery pulse is connected across LTX2, which is low resistance path. Lots of current.

Induced current in LTX1 travels through two 10K resistors, quite high resistance. Tiny current?

Even though voltage is large across both coils, the current is not the same in both coils, is it? I would think LTX2 has big current, LTX1 just small.

Target sees changing magnetic field from changing current, not voltage. How do you get twice magnetic field because current contribution from LTX1 (induced current) just a trickle -- no?

I probably misunderstand circuit, need to get LTSpice going.

Can you explain?

Thanks,

-SB

Another thought: It seems maybe what you are doing is really impedance matching through a transformer. You generate higher voltage and couple to higher resistor. But reflected back through transformer, it looks like smaller resistor to main current. Instead of using tapped auto-transformer, imagine normal transformer, what do equations tell you? I will need to see spice sim I guess.

[Carl-NC]

This issue seems fundamental to the workings of the PI MD. Is it really in question? Carl's Hammerhead article states it is the collapsing magnetic field that makes the eddy current -- is this not accepted?

I don't think Aziz is disagreeing with this. Yes, it is dB/dt that induces eddy currents in the target. And it is di/dt that produces dB/dt. I think our only disagreement is with the flyback voltage... it is an artifact of the system.

If Aziz is seeing a higher flyback that is settling faster, then I expect it is due to a faster di/dt and the arrangement of the mutually inductive coils. I doubt it is due to the higher voltage burning off the current faster. I think it is this cause & effect where we disagree.

But that's OK. The main thing is, does it Really Work? I will have an answer tomorrow. I am (right now) winding the two coils -- one center-tapped & one not -- to compare this method with the std mono. I have the mono wound and hot-glued into a TDI shell, will have the CT coil done tonight. Then I will put them on my HH board and see.

Stay tuned...

- Carl

[simonbaker]

Gentlemen, allow me to disagree.
I say it is the expanding magnetic field produced by the coil current, that kicks the target. The Flyback is actually reducing the eddy currents in the target.
The Flyback is too short to "saturate" for want of a better word, the target.
The Flyback induces currents of opposing polarity since the magnetic field of the Flyback expands and then collapses, that is, the field cuts across the target conductor first the one way, the the other way.
The current raising in the coil during TX, generates an expanding magnetic field that cuts across the target \ conductor and induces currents within the target. The first currents are superficial skin effect currents. It takes time for the eddy currents to expand to the core of the target.
How much time? The time needed has been named the TC of the target. High conductive and large size targets have long TC's. Large silver coins can have a TC of hundreds of uS.
If the TX pulse is short, the eddy currents will not have the time to expand to the very core of the target, so they will not reach maximum amplitude.
Now look at the Flyback. It's duration is a matter of nano seconds for the one way and a few micro seconds on the return. Its highest power is at the maximum voltage point that is about even both ways and of a duration of nanoseconds. Just too short to induce much eddy currents. And going both ways, thus canceling the eddy currents induced by the going field, with the field collapsing.
However, the eddy currents induced by the TX coil current persist for some time.
How much time? about the same time it took to generate them The TC of the charge curve equals the TC of the discharge curve.

So, how will I prove my point? I will show you that I can read the eddy currents without having a Flyback.
Better than that, I will show you that, without the Flyback, the amplitude of the eddy currents is much higher.
I will pulse the coil with a usual square wave TX pulse but will modify the circuit to eliminate the Flyback.
Below is the scope picture of the coil current.
The pulse length is about 64us, you can see a slight kink and noise spike where the TX switches OFF. Forgive me for the noise spike, I don't think it can be called a Flyback.
The scope is set at 20us \div and 10mV\div.
Note, this is the coil current, not the TX voltage wave form.

Please allow me some time to setup an RX circuit to read the eddy current. Just the preamp.
I will use US$ 1c, 5c, 10c, 25c as test targets.
Monolith

This issue seems fundamental to the workings of the PI MD. Is it really in question? Carl's Hammerhead article states it is the collapsing magnetic field that makes the eddy current -- is this not accepted?

Can we not determine this by experiment? Would not the expanding and collapsing magnetic fields induce currents of opposite polarity in the target? Can we not detect the polarity fairly easily?

(I'm just starting to take interest in PI theory, I hope you'll bear with my beginner questions)

Regards

-SB

[Aziz]

What do you do with the target control switch (TGTSW)? Is that just for testing without the target?

-SB

Yes, it will be switched on on every second period. This means, target will be presented to the coil on the short circuited switch. I can see then the difference of the target response.

Aziz

[Aziz]

Some hints for experimentators:

To get the proof of the earlier decaying, keep in mind, that the center-tapped coil will produce twice more signal levels! This means, that the target signal is already inherently amplified with factor of 2. This applies also to the decay curve.

To compare the proposed design with conventional design, use a conventional design with equal conditions but amplifing the RX signal twice. Then exact timings can be measured and compared.


Aziz

[simonbaker]

Hello simonbaker,



yes of course. I will provide different coil configurations with the whole projects file. This is at the moment too fresh and hot to publish. I have to tidy up my models. Maybe next weekend.
I am using LTSpice IV.
So you and others can make some experiences with LTSpice in the mean time.
Aziz

What do you do with the target control switch (TGTSW)? Is that just for testing without the target?

-SB

[Aziz]

Hello simonbaker,

Hi Aziz:

Can you post your LTSpice files and any auxiliary files/models needed to work with it? I would like to see it in action!

Also some instructions on how to install any auxiliary components, I am a little slow with understanding LTSpice setup stuff though I'm using it basic.

Good work as always!

Cheers,

-SB

yes of course. I will provide different coil configurations with the whole projects file. This is at the moment too fresh and hot to publish. I have to tidy up my models. Maybe next weekend.
I am using LTSpice IV.
So you and others can make some experiences with LTSpice in the mean time.
Aziz